<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <!-- 2，页面输入数字，点击按钮用函数求阶乘 -->
    输入数字:
    <input id="n" type="text" />
    <input type="button" value="  结果：" onclick="fn()">
    <input id="result" type="text">
    <script>
        function factorial(n) {
            if (n == 0 || n == 1) {
                return 1;
            }
            return n * factorial(n - 1);
        }
        
        function fn() {
            var num = factorial(parseInt(document.getElementById("n").value));
            document.getElementById("result").value = num;
        }
    </script>
</body>

</html>